1. Field of the Invention
The present invention relates to novel heat-stable polymers, and, more especially, to certain novel bisimido copolymers.
2. Description of the Prior Art
There have already been described in this art polymers obtained by reacting an N,N'-bisimide of an unsaturated dicarboxylic acid such as, for example, an N,N'-bismaleimide with a diprimary diamine (French Pat. No. 1,555,564). The amounts of N,N'-bisimide and of diamine are selected such that the ratio: ##EQU1## is at least equal to 1; it is generally preferred, furthermore, that it should be less than 50. Heat-stable resins are obtained which withstand severe thermal stresses remarkably well.
The abovementioned French patent also indicates that the preparation of these resins may be carried out in bulk, by heating the reactants which have previously been subjected to intimate mixing or, alternatively, in an inert polar diluent, such as dimethylformamide, N-methylpyrrolidone, or dimethylacetamide, it being possible to employ this latter process when, for example, ultimate application of the polymer mandates use of a solution thereof.
Finally, it is mentioned that for many uses it is advantageous to carry out the polymerization in two steps; in a first stage, a prepolymer is prepared by heating the intimate admixture of the two reactants to a temperature on the order of 100.degree. C. to 250.degree. C. The prepolymer obtained may be used in the form of a solution, suspension, powder, or, alternatively, it may also be shaped by merely being cast hot. In a second stage, the curing of the prepolymer may be effected by heating same to temperatures on the order of 350.degree. C., under pressure if desired.